1. Technical Field of the Invention
The present invention relates to stators for electric rotating machines that are used in, for example, motor vehicles as electric motors and electric generators.
2. Description of the Related Art
Conventionally, there are known stators for electric rotating machines which include a hollow cylindrical stator core and a stator coil mounted on the stator core.
The stator coil is formed by joining a plurality of electric wires. Each of the electric wires is comprised of an electric conductor and an insulating coat that covers the outer surface of the electric conductor.
Specifically, in forming the stator coil, each corresponding pair of the electric wires is joined by: (1) stripping the insulating coat from an end portion of each of the electric wires; and (2) welding together the exposed electric conductors of the end portions of the electric wires.
However, as shown in FIG. 23, with only the end portions 67A of the electric wires 50A stripped of their insulating coats 68A, when the end portions 67A are arranged parallel and closest to each other, there will be a gap C therebetween; the size of the gap C is equal to twice the thickness of the insulating coats 68A. Consequently, due to the gap C, the welding of the end portions 67A may result in a poor weld therebetween.
To solve the above problem, Japanese Patent Application Publication No. 2002-95198 discloses a technique according to which each of the electric wires includes a step portion formed adjacent to the end portion. Consequently, with the step portions of the electric wires, it is possible to arrange the end portions of the electric wires parallel to each other without a gap therebetween (see, seventh embodiment and FIG. 18 of the patent document).
However, to form the step portions, it is necessary to bend each of the electric wires twice in the vicinity of the end portion. Consequently, the shape of the electric wires will become complicated and the manufacturing cost of the electric wires will be considerably increased.
Moreover, according to the technique disclosed in the above patent document, the end portion of each of the electric wires is plastically deformed so that the cross-sectional area of the end portion is reduced to become smaller than that of a main portion of the electric wire. Consequently, with the reduced cross-sectional areas of the end portions of the electric wires, it is possible to reduce the amount of heat transmitted to the electric wires during the welding of the end portions, thereby preventing the insulating coats of the main portions of the electric wires from being damaged by the heat.
However, with the reduced cross-sectional areas of the end portions, the electrical resistances of the end portions are accordingly increased. Consequently, when the stator is used in a high-voltage electric rotating machine, the end portions of the electric wires will generate a large amount of heat during operation of the machine. As a result, the insulating coats of the main portions of the electric wires may be damaged by the heat generated by the end portions.